The present invention relates to a composition which is useful for performing ion-exchange chromatography. More particularly, the present invention relates to an improved composition and method for preparing an ion-exchange composition in which resin support particles are irreversibly attached to fine resin layering particles via a dispersant material. The composition of the present invention can be used in conventional column chromatography but is most particularly suited for high performance liquid chromatography.
Generally, ion-exchange chromatography employs the use of columns. These columns are packed with a resin, often in the form of granules having sorptively active surfaces or surfaces which have been coated with a substance which is sorptively active.
It is well recognized in the art that excellent chromatographic supports consist of a plurality of discrete particles of regular shape, preferably spheres, having surfaces with a large number of superficial shallow pores. In order that columns will give reproducible chromatographic results, support granules ideally should be regular in their surfaces and their surface characteristics easily reproducible.
Materials for performing liquid chromatographic analyses are known where only the thin outer surfaces of the chromatographic support materials are available for actively exchanging ions with liquid media. For example, Small et al., in U.S. Pat. No. 4,101,460 (1978) describes the preparation and use of an ion-exchange composition comprising Component A, an insoluble synthetic resin substrate having ion-exchanging sites, at least on its available surface, and having Component B, a finely divided insoluble material, irreversibly attached thereto.
Small et al., in U.S. Pat. No. 4,252,644, describes the process for chromatographic separation of ions of like charge using the ion-exchange composition described in U.S. Pat. No. 4,101,460.
Iler, in U.S. Pat. No. 3,485,658 (1969), describes the preparation of chromatographic support materials where alternating layers of colloidal solid particles are laid down on a substrate by treating the substrate with dispersions of oppositely charged colloidal particles. These colloidal particles include alumina, silica, and ionic synthetic polymers.
Kirkland, in U.S. Pat. No. 3,505,785 (1970) discloses an improved process for performing chromatographic separation by contacting the materials to be separated with superficially porous refractory particles having impervious cores. These particles have a coating of a series of sequentially adsorbed like monolayers of like colloidal inorganic microparticles irreversibly attached thereto. The cores consist of glass spheres and the coating consists of monolayers of silica.
Horvath et al. describes ion-exchange chromatography using glass beads having a styrene-divinylbenzene resin skin, sulfonated or aminated to produce cation and anion exchange materials termed "pellicular resins," which refers to the skin-like layer of active sites on these beads. Analytical Chemistry 39:1422 (1967). These skin-like layers are physically held in place. In a pellicular resin, the support bead typically has a spherical annulus configuration with a stable resin layer deposited onto the surface of the beads. Since the support particles are usually smaller than 50 .mu.m, making a uniform resin layer on their surface without agglomeration of the particles is difficult.
Hanakoa et al., in U.S. Pat. No. 4,447,559 (1984) discloses an ion-exchanger having resin support particles, a binder resin of the same or similar composition as the support resin, and fine synthetic resin particles with anion-exchange groups. Hanakoa requires amination of the binder. However, linear aminated polymer, an unavoidable by-product of binder amination, tends to agglomerate to the resin support particles. This can interfere with attachment of the fine resin particles to the binder, producing an insufficient coating of support particles by the fine resin particles.
All of the methods described above, except Horvath, et al. and Hanakoa, et al., involve a coating process in which the coating is attached to the resin support particles by electrostatic forces.
In the prior art electrostatic methods, the resin support particles are typically formed by a suspension polymerization process. A dispersant material is frequently used in polymerization of the resin support particles to maintain separate particles in the reaction solution as they are produced, preventing the desired size particles from sticking to each other and forming a larger agglomerate particle. The resin support particles are then lightly sulfonated by exposing them at room temperature to concentrated sulfuric acid for a few minutes. This creates a very thin layer of sulfonate sites on the surface of the resin support particle and allows for electrostatic attachment of fine resin layering particles such as aminated latex beads.
Thus, in the prior art electrostatic methods it is necessary to functionalize the fine layering particles by creating a positive or negative charge, at least at the surfaces of those particles, for the electrostatic attachment. This has been done by aminating or sulfonating latex-derived particles. If the latex is aminated, the resin support particles are sulfonated. In the case where the latex is sulfonated, the resin support particles are aminated. The latex and support particles are then brought into contact with each other, resulting in a monobead coat of latex particles electrostatically attached to the surface of the resin support particles. This produces a pellicular anion-exchange or pellicular cation-exchange resin bead.
The above-described methods of electrostatically attaching the synthetic fine resin layering particles to the resin support particles have many disadvantages. First, it is difficult to control the resin sulfonation step so that the degree of sulfonation is sufficient to allow complete coverage of the resin support particle with the electrostatically-attached latex, without having any residual cation-exchange capacity on the support particles. The residual cation-exchange capacity can lower the efficiency of the column.
In addition, linear aminated polymer, which is an unavoidable by-product of aminated latex preparation, tends to agglomerate to the resin support particles due to much higher diffusion rates over the much larger latex particles. The agglomeration of linear polymer can interfere with attachment of the latex particles to the resin support particles, producing an insufficient coating of the support particles by the latex particles. The result is a much lower than expected and non-reproducible ion-exchange capacity. Moreover, if oligomers or linear polymer is left in the resin support particle mixture after polymerization, when the prior art method of sulfonation of support particles is performed, these by-products are also sulfonated and are then free to interfere with the newly attached anion-exchange latex after latex agglomeration. This results in a low ion-exchange capacity as well.
In addition, some compositions cannot be agglomerated prior to packing of a column because they cannot withstand packing pressures. Therefore, the resin layering particle solution must be pumped through the packed column bed. With the electrostatic attachment method of the prior art, the column yield is poor due to unbalanced charge interaction between resin layering particles and resin support particles, thereby disrupting the packed bed.
The prior art methods of electrostatically attaching fine resin layering particles, such as latex, to resin support particles have additional limitations. In the prior art methods, it is necessary to maintain a dispersed latex suspension to prevent attachment of latex agglomerates to the resin support particles through the latex functionalization step. This limits the possible chemistries of functionalization to those that can be carried out in an aqueous system and which will not disrupt the latex suspension, thereby generating large clumps of agglomerates. Many of the larger, more aliphatic amines are not very water soluble and amination is normally carried out in a mixed water/solvent or nonaqueous system. These compounds cannot be used with the prior art method of electrostatic latex attachment.
In addition, it is not possible to use chemistries on agglomerated latex electrostatically attached to a support substrate that will reverse the charge on the latex, since the latex will no longer remain attached.